1. Field of the Invention
This invention relates to a coulometric device for performing integration and timing functions with precise end-state detection.
2. History of the Art
Coulometric devices for performing time integration are disclosed in U.S. Pat. No. 3,045,178 issued to Lester Corrsin on July 17, 1962. Such devices typically comprise a body of electrically non-conductive material having a bore therethrough which supports two columns of mercury, the adjacent inmost ends of which are separated by, but maintained in contact with a small volume of liquid electrolyte. The outermost ends of the two metal columns are maintained in contact with suitable conductive leads provided to connect the instrument to a source of electrical current. The flow of electrical current from one metal column to the other through the electrolyte causes metallic ions to migrate from the positive column (anode) to the negative column (cathode). In accordance with Faraday's law, the liquid metal is electroplated from the anode column to the cathode column, causing the anode column to decrease in length and the cathode column to elongate correspondingly, the change in column length being directly proportional to the total electric charge passed through the device. When such a device is connected to a source of constant direct current, readout of the measured time-current product may be effected by comparison of one column length against a calibrated scale.
In the operation of such a device, it is frequently desirable to detect electrically the end state of device operation wherein one of the electrodes penetrates the moving electrolyte gap and finally bridges it, shorting out the electrolyte. U.S. Pat. No. 3,462,684 issued to Curtis C. Beusman, discloses one such detection scheme wherein an a.c. voltage source is connected across the coulometer and an a.c. voltage detector is provided to measure the a.c. voltage drop across the coulometer. End-state operation is detected by a reduction in the a.c. voltage drop across the coulometer.
While the a.c. end-state detection is useful in many applications, it is subject to certain inherent inaccuracies. It has been found very difficult to construct a coulometer with characteristics that are precisely matched for both positive and negative current flow. Impurities and non-homogeneities can cause impedance to appear different for positive current flow than for negative flow, with the consequence that high amplitude a.c. signals can produce a finite d.c. component in series with the coulometer. This d.c. component can produce a measurable error in the coulometer readout.
Accordingly, there is a need for a coulometric device provided with means for precise end-state detection.